PROJECT SUMMARY ? PROJECT 2 We are proposing the creation of a research program entitled, ?Increasing the therapeutic index of brain tumor treatment through innovative FLASH radiotherapy (FLASH-RT), focused on translating a novel irradiation (IR) modality rapidly into the clinic. The overall hypothesis to be tested is whether radiation delivered at ultra high dose rates (compared to the much lower dose rates used in current clinical practice) can significantly ameliorate normal tissue complications while maintaining acceptable if not improved tumor control. To test this hypothesis, the program will deploy a comprehensive series of preclinical studies that will critically evaluate tumor control, neurocognitive outcomes and resultant radiation injury to the brain following FLASH-RT and conventional dose rate irradiation. Collectively, these studies will generate the requisite data sets required for the rapid translation of the novel FLASH irradiation platform to the clinical scenario. Preclinical studies in mice assessing orthotopic tumor control, cognition, neuronal and vascular structural plasticity, immune-modulation and oxygen dependent mechanisms of radiation injury are coupled with a clinical trial in GBM dog patients to inform the oncologists of the potential benefits of this potentially paradigm shifting technology. The objectives of this program project will be facilitated by the activities conducted by the dosimetry/physics core (Core 2) and the Neurobehavioral Core (Core 3). The current project (Project 2) involves a comprehensive series of cognitive, structural and immunological studies that will elucidate the long-term (up to 6 months) impact of clinically relevant brain tumor treatments on cognition under carefully controlled conditions. Animals subjected to single fraction IR, multifraction IR, and multifraction IR +/- TMZ treatment arms will be behaviorally tested then analyzed for some of the critical neuronal and vascular changes essential for proper cognitive function. For this project, these proof-of-principal, long-term neurocognitive studies must be made in tumor free animals in the absence of confounding disease, and will provide the foundation for elucidating the basis normal tissue sparing afforded by FLASH in the irradiated brain.